The present invention relates to a process for reactivating spent aromatics alkylation catalysts, and in particular the spent catalysts used in the alkylation and transalkylation steps of the liquid phase processes for the production of ethylbenzene and cumene.
Ethylbenzene and cumene are valuable commodity chemicals which are used industrially for the production of styrene monomer and phenol respectively. Ethylbenzene may be produced by a number of different chemical processes but one process which has achieved a significant degree of commercial success is the vapor phase alkylation of benzene with ethylene in the presence of a solid, acidic ZSM-5 zeolite catalyst. In the commercial operation of this process, the polyalkylated benzenes, including both polymethylated and polyethylated benzenes, which are inherently co-produced with ethylbenzene in the alkylation reactor, are transalkylated with benzene to produce additional ethylbenzene either by being recycled to the alkylation reactor or by being fed to a separate transalkylation reactor. Examples of such ethylbenzene production processes are described in U.S. Pat. Nos. 3,751,504 (Keown), 4,547,605 (Kresge), and 4,016,218 (Haag).
More recently focus has been directed at liquid phase processes for producing ethylbenzene from benzene and ethylene since liquid phase processes operate at a lower temperature than their vapor phase counterparts and hence tend to result in lower yields of by-products. For example, U.S. Pat. No. 4,891,458 describes the liquid phase synthesis of ethylbenzene with zeolite beta, whereas U.S. Pat. No. 5,334,795 describes the use of MCM-22 in the liquid phase synthesis of ethylbenzene.
Cumene has for many years been produced commercially by the liquid phase alkylation of benzene with propylene over a Friedel-Craft catalyst, particularly solid phosphoric acid or aluminum chloride. More recently, however, zeolite-based catalyst systems have been found to be more active and selective for propylation of benzene to cumene. For example, U.S. Pat. No. 4,992,606 describes the use of MCM-22 in the liquid phase alkylation of benzene with propylene.
Other molecular sieves known for use as liquid phase alkylation and transalkylation catalysts include MCM-36 (see U.S. Pat. No. 5,258,565), MCM-49 (see U.S. Pat. No. 5,371,310) and MCM-56 (see U.S. Pat. No. 5,453,554).
Although MCM-22 and the related molecular sieves MCM-36, MCM-49 and MCM-56 are uniquely resistant to deactivation by coking, when used in liquid phase alkylation and transalkylation processes, they are susceptible to deactivation as a result of poisons, particularly nitrogen and sulfur compounds, in the feeds. In the past, this has required periodic ex-situ regeneration of the catalyst by contacting the spent catalyst at elevated temperature with flowing air so as to remove the deactivating species and burn off any coke deposits. However, although such air regeneration is effective in improving the activity of the catalyst, it tends to be accompanied by a decrease in the monoalkylation selectivity of the catalyst. This results in a significantly increased duty on the transalkylator and a consequent drop in overall yield and product purity. There is therefore a need for a regeneration protocol which minimizes this change in the selectivity of the catalyst to undesirable by-products.
According to the invention, it has now been found that contacting the air-regenerated catalyst with an aqueous medium is effective in restoring the selectivity of the catalyst back to its fresh state. In one desirable embodiment of this concept, the washing solution is an aqueous solution of ammonium nitrate or ammonium carbonate. In another desirable embodiment of this concept, the washing solution is an aqueous solution of acetic acid.
In one aspect, the invention resides in a process for regenerating a spent aromatics alkylation or transalkylation catalyst comprising a molecular sieve, the process comprising the steps of contacting the spent catalyst with an oxygen-containing gas at a temperature of about 120 to about 600xc2x0 C. and then contacting the catalyst with an aqueous medium.
Preferably said aqueous medium is selected from the group consisting of an ammonium nitrate solution, an ammonium carbonate solution and an acetic acid solution.
Preferably, the step of contacting the catalyst with an aqueous medium is conducted at a temperature of about 15 to about 120xc2x0 C. for a period of about 10 minutes to about 48 hours.
Preferably, after contacting with the aqueous medium, the catalyst is calcined at a temperature of about 25 to about 600xc2x0 C. for a period of about 10 minutes to about 48 hours.
In a further aspect, the present invention resides in a process for alkylating an aromatic compound comprising the steps of:
(a) contacting an alkylatable aromatic compound and an alkylating agent with an alkylation catalyst comprising a molecular sieve under alkylation conditions;
(b) when said alkylation catalyst has become at least partially deactivated, contacting said alkylation catalyst with an oxygen-containing gas at a temperature of about 120 to about 600xc2x0 C.; and then
(c) contacting the catalyst from step (b) with an aqueous medium.
Preferably, the molecular sieve of the alkylation catalyst is selected from MCM-22, PSH-3, SSZ-25, MCM-36, MCM-49, MCM-56, faujasite, mordenite and zeolite beta.
Preferably, the contacting step (a) is conducted in the liquid phase.
Preferably, the alkylating agent includes an alkylating aliphatic group having 1 to 5 carbon atoms.
Preferably, the alkylating agent is ethylene or propylene and the alkylatable aromatic compound is benzene.